STAINING-REACTIONS OF THE GERM-NUCLEI It was pointed out by Ryder in 1883 that in the oyster the germnuclei stain differently in the two sexes ; for if the hermaphrodite gland of this animal be treated with a mixture of saffranin and methylgreen, the egg-nuclei are coloured red, the sperm-nuclei bluish-green. A similar difference was afterwards observed by Auerbach ('9I) in the case of many vertebrate germ-cells, where the egg-nucleus was shown to have a special affinity for various red and yellow dyes (eosin, fuchsin, aurantia, carmin), while the sperm-nuclei were especially stained with blue and green dyes (methyl-green, aniline blue, haematoxylin). He was thus led to regard the chromatin of the egg as especially " erythrophilous," and that of the sperm as " cyanophilous." That the distinction as regards colour is of no value has been shown by Zacharias, Heidenhain, and others ; for staining agents cannot be logically classed according to colour, but according to their chemical composition ; and a red dye, such as saffranin, may in a given cell show the same affinity for the chromatin as a green or blue dye of different chemical nature, such as methyl-green or haematoxylin. Thus Field has shown that the sperm-nucleus of Asterias may be stained green (methyl-green), blue (haematoxylin, gentian violet), red (saffranin), or yellow (iodine), and it is here a manifest absurdity to speak of " cyanophilous " chromatin (cf. p. 243). It is certainly a very interesting fact that a difference of staining-reaction exists between the two sexes, as indicating a corresponding difference of chemical composition in the chromatin; but even this has been shown to be of a transitory character, for the staining-reactions of the germ-nuclei vary at different periods and are exactly alike at the time of their union in fertilization. Thus Hermann has shown that when the spermatids and immature spermatozoa of the salamander are treated with saffranin (red) and gentian violet the chromatic network is stained blue, the nucleoli and the middle-piece red ; while in the mature spermatozoon the reverse effect is produced, the nuclei being clear red, the middle-piece blue. A similar change of stainingcapacity occurs in the mammals. The great changes in the stainingcapacity of the egg-nucleus at different periods of its history are described at pp. 245, 246. Again, Watase has observed in the newt that the germ-nuclei, which stain differently throughout the whole period of their maturation, and even during the earlier phases of fertilization, become more and more alike in the later phases and at the time of their union show identical A very similar series of facts has been observed in the germ-nuclei of plants by Strasburger (p. 163). These and many other facts of like import demonstrate that the chemical differences between the germ-nuclei are not of a fundamental but only of a secondary character. They are doubtless connected with the very different character of the metabolic processes that occur in the history of the two germ-cells ; and the difference of the staining-reaction is probably due to the fact that the sperm-chromatin consists of pure or nearly pure nucleic acid, while the egg-chromatin is a nuclein containing a much higher percentage of albumin.